The present invention relates to an inductive load drive circuit, particularly for fuel injectors.
On automotive electronic injection systems, fuel supply is enabled by means of an electronically controlled valve, operation of which is controlled by the magnetic field produced by an electromagnet roughly describable as an inductor wound about a core and through which a control current is supplied.
To reduce dissipation, control is effected in two phases: a first phase requiring a strong magnetic field for opening the valve (peak phase); and a second phase in which the valve is kept open (hold phase), and in which a lower magnetic field and, consequently, a lower control current are required.
FIG. 2 shows a rough half line graph of the control current I.sub.L of an injector. As can be seen, the peak phase extends up to instant t.sub.1, with current I.sub.L increasing up to a maximum value I.sub.p. This is followed by phase t.sub.1 -t.sub.2 in which the current falls sharply, depending on application requirements; an uncontrollable phase t.sub.2 -t.sub.3 ; and, from t.sub.3 onwards, the actual hold phase, chopped to prevent active elements in a linear zone resulting in dissipation.
The passage from peak current I.sub.p to the hold current (ranging between a maximum I.sub.HMAX and a minimum I.sub.HMIN) must be effected rapidly, for which purpose provision is made for recirculating high voltage current (freewheeling zone), i.e. to reduce the current in the inductor, this is supplied with a high voltage for forcing a fall in current.
The high speed required in passing from the peak to the hold current results in magnetic problems, the effect of which is to create a zone (interval t.sub.2 -t.sub.3 in FIG. 2) that is hard to control.
As the presence of uncontrollable zones may result, in some cases, in malfunctioning or at any rate in impaired reliability of the circuit, such zones must perforce be eliminated. One known method of doing this is to maintain the high speed recirculating or free-wheeling phase until the current in the load drops to a so-called "undershoot" value, lower than that of the hold current, and to only subsequently commence the hold phase. The corresponding current pattern in the inductor is as shown in FIG. 3, wherein the recirculating phase is maintained up to instant t.sub.4, at which point the recirculating current reaches the undershoot value I.sub.UND, marking the start of the hold phase in which the current in the inductor oscillates between I.sub.HMAX and I.sub.HMIN as in FIG. 2.
The above solution, however, is also unsatisfactory, in that it requires a highly accurate I.sub.UND value at which to arrest the high speed recirculating phase, to prevent too low a current value, and consequently too low a magnetic field, from closing the valve. The attainment of a sufficiently high degree of accuracy inevitably results in difficulties (again affecting the reliability of the circuit) or at any rate in complex design and high manufacturing cost of the circuit.